Failure of the SCR (Selective Catalytic Reduction) system leads to excessive nitrogen oxide (NOx) emissions and may trigger protective measures such as reduced engine performance, illumination of the malfunction indicator lamp (MIL), vehicle speed limitations, or a no-start condition.

The SCR system functions by injecting a urea solution (DEF) into the exhaust stream, utilizing a catalyst to convert harmful NOx into harmless nitrogen gas (N₂) and water (H₂O). When the SCR system malfunctions, common consequences include:

**Excessive NOx Emissions:** The core function of the SCR system is to reduce NOx emissions. Once it fails, the nitrogen oxides in the exhaust cannot be effectively reduced, directly resulting in emissions exceeding regulatory limits and violating environmental regulations.
**NOx Sensor Alerts:** The system relies on upstream and downstream NOx sensors to monitor conversion efficiency. If the detected efficiency falls below a specific threshold, it triggers diagnostic trouble codes (DTCs), such as P20EE or P207F.
**Reduced Engine Power (Limp Mode):** To prevent further environmental pollution, the vehicle's control system may activate "Limp Mode," limiting the engine's power output and thereby compromising driving experience and safety.
**Chain Reactions from Upstream Faults:** The SCR is the final stage of the exhaust aftertreatment system; its failure is often caused by anomalies in upstream components (such as the DOC, DPF, EGR, or turbocharger). If these upstream issues are not diagnosed and resolved first, simply replacing the SCR unit may not provide a complete or permanent repair.
**Catalyst Coating Damage or Blockage:** Prolonged use of poor-quality fuel, the ingress of engine oil into the exhaust system, or the accumulation of crystallized urea deposits can damage the internal coating of the SCR catalyst or clog its internal channels, thereby impairing its reaction efficiency.